This is built for convenience and without any warranties. It's also built on top of the official library/ruby:2.1-alpine container which at the time of creation already had critical vulnerabilities.
Quickstart for current version:
Use something like this:
docker pull frumpel/jekyll-3.0.2:0.2
docker run -it --rm -v $(pwd):/jekyll-src -v $(pwd)/_site:/jekyll-dst -p4000:4000 frumpel/jekyll-3.0.2:0.2
which will in effect run
jekyll serve --watch